1. Field of the Invention
The present invention relates to the field of non-destructive testing, and, more particularly, to the field of non-destructive testing using eddy current measuring systems which rely on the interaction of time varying electromagnetic fields with a conductive specimen under test.
2. Discussion of the Prior Art
The use of eddy current techniques for non-destructive testing of conductive specimens is well known in the art. Typically, systems employing eddy current techniques generate electromagnetic fields at a probe which induce eddy currents in a test specimen, which set up their own electromagnetic field distribution which couples with the initial excitation field produced by the probe coil. This coupling effectively transfers the impedance of the test specimen to the probe excitation coil so the changes in the test specimen which affect the transfer impedance may be detected at the excitation coil. The transfer impedance is affected by a number of factors including the conductivity properties of the test specimen, magnetic permeability, and geometrical factors such as proximity of the excitation coil to the specimen.
Generally, non-distructive test systems using eddy current techniques have been largely used for defect detection and characterization, and for materials sorting and sizing applications. However, any material characteristic which affects the transferred impedamce, either directly or indirectly, can be detected and measured by the eddy current probe. Thus, apart from the obvious direct measurement of speciment conductivity and magnetic properties, eddy current techniques have also been used to measure corrosion thickness, hardness, heat treating, and residual stress levels. Measurement of such mechanical properties are indirect in that the property change affects the metallurgical behavior of the material which results in changes in conductivity and permeability, thereby in turn alternatinng the measured impedance of the eddy current test probe.
Eddy current techniques for non-destructive testing rely on an accurate measurement of probe impedance as this is the parameter which is directly affected by defects or changes in material properties. At present, there are two major techniques by which eddy current probe signals are acquired and analyzed, respectively employing impedance bridges, and quadrature synthesis. However, neither of these techniques provides a direct indication of probe impedance, but only provides measures of relative changes in the probe impedance with location or time.
In addition, with impedance bridge techniques, all processing is performed with analog circuits, thus reducing accuracy and lowering the signal-to-noise ratio of the final result.
Quadrature synthesis techniques do not use an impedance bridge, but require the creation of two quadrature phase signals representing the voltage waveform driving a probe. Various combinations of the two phase shifted waveforms are used with an output signal from the probe to create signals for driving a display device which displays relative impedance changes. Like the impedance bridge technique, an output signal is provided which is proportional to a change in probe impedance, but it is difficult to map an actual impedance value back to a known defect.
An additional problem with both impedance bridge and qaudrature synthesis techniques is that a nulling circuit is required to each, which adds to the circuit complexity and provides additional circuitry which may affect the results and the reliability thereof.
Still further, it is difficult to use the balanced bridge and quadrature synthesis techniques with a plurality of probes, or with probes operating at different frequencies, and the speed with which relative impedance values are determined is rather slow. For different probes, it is even slower as rebalancing for each probe is required.